Many displays useful for presenting information are based on light emitting diodes, including organic light emitting diodes (OLED) and active matrix organic light emitting diodes (AMOLED).
Typically, drive voltages of OLEDs rise with time during operation. The OLED's brightness tends to decay and the voltage variation of the node which connects OLEDs to their driving thin film transistors (TFT) also affect the operation of that TFT.
Once the voltage of the TFT varies, e.g. its drain or source voltage, the driving current of the TFT decreases under the same grey-level data input. This all tends to decrease the brightness of the display over time.
Referring now to FIG. 1, in typical methods for driving OLED displays, the system power VDD is always higher than VSS, because typical OLEDs can be turned on only in this state. Further, VG, the gate voltage of a TFT, is always between VDD and VSS. In this state, the TFT will accumulate electric charges gradually and the lifetime of the OLED will concurrently be decreased gradually. However, this method is not desirable for use with displays using AMOLED displays.